2 Answers
2

The specific timeline is never revealed, but it is within a few weeks.

Bane and Pavel tell everyone publicly that the bomb will take 6 months to deteriorate by itself. Lucious Fox, however, explains that the core will overheat and detonate in 54 days if it is not reconnected to the full generator. The number of days remaining can be seen on the countdown timer on the core.

While R'as Al Ghul is typically associated with the Lazarus Pit, I believe the prison depicted is meant to be a reference to Santa Prisca, where Bane was born in the comics.

But didn't J. Gordon Levitt or somebody talking to him mention that already 3 months passed at some point during the occupation?
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Tom Cody♦Jul 26 '12 at 19:22

Not that I recall, but I may have missed it. Anyone else remember for sure?
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phantom42Jul 26 '12 at 19:26

I saw the film last night and someone definitely said "3 months" - however I am sure Wayne was still in prison at that point. Was the "54 days" you mentioned before the core was removed, or later on (e.g. in the 3 months part)?
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DisgruntledGoatJul 29 '12 at 18:52

1

The time Bane mentions is right. Bruce Wayne returns sometime in the winter, and all the events before the mentioned fight occur during the fall or even summer(remember the way people were dressed). I remember the time was 5 months, but I can't come with a quote.
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DragosJul 30 '12 at 11:40

1

Logically we can assume the bomb was started in late Sept - Oct since football starts in August however most season games do not start until after Labor Day. Also the way people where dress since Gotham is more or less NYC. So assuming the same climate the way people where dress would point towards that time frame. As for the time of batman's return it would have to be Jan, Feb for the river to be frozen all the way across.
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OrionDarkwoodJul 31 '12 at 15:27

It was 3 months- I am watching it right now and Joseph Gordon Levitt's character is talking to the cops/agents who came with the emergency food shipment with the commissioner in the room as well. They are talking about the 3000 cops in the tunnels who "haven't seen daylight in 3 months." Which makes sense because as a previous poster said football season is in the early fall and was just beginning to snow so that means it is around early December.